Tube Cutting Templates
The classic 4130 chromoly steel welded structure has always been one of the most common building mediums to work with on experimental aircraft. This type of construction lends itself to a multitude of different types of applications and renders one of the highest strength to weight ratio manufacturing techniques, especially when it comes to fuselage assemblies. The welding of steel tube assemblies is a process that can be readily learned by just about anyone. And with current welding technologies like the TIG (tungsten inert gas) welder now coming down in price and becoming readily available to the average builder, precision welded aircraft subassemblies are no longer relegated to the professionals. (Figure: 1) Although this article is not a treatise on welding techniques, it is the primary answer to “How do I become a good welder?”
Becoming a good welder requires that you learn the principles of welding. Our recommendation, especially if you’re brand new to welding, is that you simply engage in a training program. Often a community college class is your most cost-effective method of learning the skills you need. And then, of course, practice is the key to becoming proficient. As you begin the process of welding, one of the first things that you will identify is that it becomes very easy to make beautiful looking welds if everything is set up properly. Good equipment, good environment, clean materials, and, equally as important, a proper fit of the pieces of material which you’re welding together. This has always been one of the most frustrating parts of making a 4130 chromoly steel fuselage assembly. Typically, when we are working off of a set of plans, we are taking a piece of 4130 tubing, cut it to length, and then grinding each end to precisely interface with the adjacent tube. We refer to this as “coping”. This process is usually a lengthy process of trial and error. We place the tube in position, then mark it, and then grind the end of the tube, refit the tube in place, check it, market and duplicate the process all over again until we have a proper fit. The process can be tedious, but if you have patience and a good eye for spatial orientation, with a little bit of practice, you can become pretty good at the process. All this being said, I’ve never met anyone who has welded a steel fuselage frame who has not come across the issue of fitting the tube and ending up with a fairly large gap on accident. If you’ve ever tried to close up that 1/4-inch gap by welding, you know that the end result isn’t going to be all that pretty. Those really pretty welds, that we all admire, are primarily a result of having two pieces of metal properly prepped and with a very nice clean consistent fit against each other. The welding bead flows very seamlessly and consistently because of this close contact. Producing a beautiful weld with these conditions is a no-brainer.
So the question is, if the difference between a good and a mediocre weld is the fit, how do we produce a consistently reliable good fit? The importance of a good fit is so critical that the industry in general has gone to great lengths to try and solve this age-old problem. There have been literally hundreds of CNC machines manufactured to be able to deal with exactly this problem. However, for the average homebuilder a $150,000 CNC tube cutting machine isn’t exactly a good fit. And even the cheap CNC machines at $40,000 are still a ridiculous option. Those that have purchased the CNC tube cutting machines typically offer their services to precut your tubing for you using their machine. And although an aircraft manufacturer who is building a multitude of the same frames over and over can make use of these services, the price for this service to the average homebuilder is still not cost-effective. There is also a myriad of different types of tube coping machines using a drill press and a fixture to hold the tubing in place while cutting the profile with a hole saw. We, personally, own three of these. Today, they all sit in the bottom of a toolbox somewhere. The problem with all of these methods has to do with the special nature of the 4130 chromoly steel fuselage frames used in aircraft. Most of the tubing, which we’re using, is thin wall tubing with a thickness of, typically, .035”. Unless you are buying hole saws with about 48 teeth per inch, it is just a brutal process cutting through this thin wall steel tube. Even when working with thicker wall tubing, the cutting process can work ok, but the downside is the set up time can be quite frustrating when moving from one size tube to the next. After years of dealing with these problems, and having built a myriad of steel fuselage assemblies, we finally created a better mousetrap: tube cutting templates.
Many years ago, we started doing most of our design work using SOLIDWORKS 3D modeling software. Within the software, there is the ability to be able to generate automatic tube profiles using basic line geometry. (Figure: 2) the process is very similar to how a lot of drawings were created in the earlier days of aircraft design simply using the basic geometry as a centerline for each piece of tubing. In the SOLIDWORKS environment, we take each one of the lines and assign it a tubing profile called a “weldment”. We have created a weldment profile database for each of the tube sizes which we use and can now simply select a line and assign a tube size. Building a 3D model of the frame now becomes a breeze. When you assign a weldment profile to a line it creates a tube the exact length of that line. This creates a tube longer than necessary that extends to the intersection of the lines. The next built-in feature, that is really helpful, is the “trim” tool. (Figure: 3) It allows us to cope the end of each tube to perfectly match the profile of any adjacent tubes. There are a multitude of different trim combinations that can be selected and even a selection for the gap size between the tubes. We usually work with a .005” gap when working with thin wall tubing. Each one of the tubes within a frame will require a different end treatment depending on the sequence of the build and other structural requirements. In addition to being able to cope the ends of each tube, we can also create intersections within the mid span of the tube. Once we have created all of the interactions with the other tubes, we can simply take the individual tube out of the frame and create its own separate part. We can then manipulate that part as its own entity. And now is where the magic really comes into play. We can take the tube profile and cut a slit down the entire length of that tube. This now becomes, rather than a tube, a curved selection which we can treat as a piece of sheet metal. SOLIDWORKS allows us to now flatten out that curved piece of sheet metal and turn it into a flat template (Figure: 4). We can then take this template and create a drawing at 100% scale which we can print out on a home computer. Once printed on a piece of paper, we have built in testing dimensions to ensure that our printer is printing truly at 100% scale. We can then cut out the template with scissors or an X-Acto knife and we have a paper template which we can wrap around the perimeter of a piece of tubing. We start with a tubing “blank”. A blank is simply a specific sized tube diameter, wall thickness, and overall length. These dimensions are labeled on the tube template. In order to mark tubes that are longer than what will fit on a standard 8.5 x 11 sheet of paper, we have created templates with breaks along the length of the drawing. This allows us to have all of the critical information on one single piece of paper. When dealing with a longer piece of tube, we start off by drawing a line down the length of the tube that we use as a reference mark to align the edge of the template. This is easily accomplished by using a piece of angle, or channel, placed directly onto the tube creating a self-aligning straight edge.(Figure: 5)
We can then slide the template to predetermine dimensions specified on the template for making additional marks that can be used for identifying additional cutouts or other tube intersections. And the final tube end coping layout can be obtained by simply sliding the template to the other end of the pre-cut tube, and marking with a sharpie or magic marker. Because the tube template is slid right to the end of each end of the precut tube the amount of material necessary to grind is minimal. We have several CNC machines which we have set up in the past to cut steel tube profiles. Even in a mass production environment, where we are making twenty of the same tubes at a time, we still find the tube template process more efficient. It’s great when you develop a system by where the most efficient process is also the least costly.
All you need to take advantage of this system is a magic marker, and a small bench grinder. Probably the most amazing part of the tube template system is the consistent precision fit of each one of the tubes. It’s so fun to weld when everything lines up perfectly. This process is so consistently accurate it makes even the newest of welders capable of making professional looking welds. On more than one occasion, we’ve had individuals tell us that we should be doing this system as a business plan to make templates for other aircraft manufacturers. And although our interests lie elsewhere, we believe that this could very easily become the new norm for plans built or kit planes. If you’re interested in seeing more of this system, all of the plans including the tube marking templates for the EMG-6 electric motor glider, as well as some generic templates that you may be able to use on your aircraft are available free on the Adventure Aircraft website. More importantly, with EAA’s new program to make SOLIDWORKS available for free to its membership, you now have the opportunity to create your own tube marking templates for the aircraft that you’re working on.